1. Field
Embodiments of the invention relate generally to a method for manufacturing a nonvolatile storage device.
2. Description of the Related Art
It is considered that flash memory widely used as a nonvolatile storage device encounters limitations on the improvement in integration degree. A crosspoint nonvolatile storage device having a configuration in which a storage layer (storage unit) variable in electric resistance is placed between two electrodes, for example, receives attention as a nonvolatile storage device capable of achieving a higher integration degree than flash memory (e.g. JP-A 2007-184419 (Kokai)). There is also a nonvolatile storage device of three-dimensional structure in which the crosspoint nonvolatile storage devices are stacked.
In the manufacture of such a crosspoint nonvolatile storage device, a storage layer that forms memory cells, for example, is processed into shapes corresponding to bit lines; an interlayer insulating film is buried therebetween; then a metal film for word lines is stacked thereon; and word lines are formed by performing dry etching processing using a mask of a silicon oxide film, for example, which is processed into shapes corresponding to the word lines by photolithography.
At this time, during the processing of the storage layer, a taper that has a larger area in the lower portion of the storage layer than in the upper portion thereof may be created. Burying the interlayer insulating film between the storage layers in this state forms a configuration in which the interlayer insulating film covers the storage layer at the interface between the interlayer insulating film and the storage layer. In this state, if processing that forms the word lines is performed, the storage layer that has not been processed under the shade of the interlayer insulating film is left between the word lines above the bit lines, and this causes the problem of a short between the word lines.
For example, when etching processing of a cell material is performed in a first direction in order to form the interlayer insulating film, since a trench formed by etching usually has a reverse-tapered shape, the interlayer insulating film buried in this trench also has a reverse-tapered shape. Then, when etching processing of the cell material is performed in a second direction that crosses the first direction, since the interlayer insulating film that has already been buried has a reverse-tapered shape, this interlayer insulating film forms a shade and a residual component of the cell material (e.g. semiconductor, metal, etc.) is left in the processed portion. The presence of a residue like this causes the problems of a short (electrical short circuit) between cells and a variation in characteristics of the nonvolatile storage device.